Orcas, or killer whales, have quite the reputation to live up to. Their name implies that they are violent and bloodthirsty, and many people interpret the "killer" in "killer whale" as meaning predatory toward humans. Even the gentler sounding scientific name, Orcinus orca, refers to Orcus, Roman god of the underworld.

In reality, orcas aren't whales at all, nor are they often violent toward humans. They're actually part of the oceanic dolphin family (Delphinidae), and their killer reputation reflects their status as apex predators -- some orcas even occasionally eat shark species.

In the North Pacific, there are subgroups of orcas, or ecotypes: resident orcas, transient orcas and offshore orcas, each of which has its own habits and prey. Offshore orcas live far off the coast in the northeast Pacific and have only been recognized since the 1990s; little research has been done on them. But the differences between resident and transient orcas, which we'll focus on, have been studied and are at least somewhat understood.

Resident orcas are identifiable by their dorsal fins which have rounded tips and are positioned further back than they are on transient orcas. (Photo credit: Carrie Vonderhaar)Click to enlarge

Residents and transients likely have not interbred for up to 10,000 years, and when they happen to travel through the same areas, they do not mingle, preferring to ignore or avoid each other. The differences between the two ecogroups are marked, yet according to Erich Hoyt, senior research fellow with the Whale and Dolphin Conservation Society, they're still the same species. "At some point, residents and transients may be shown to be different species," he says, "but they haven't yet."

Let's take a closer look at the differences between these two ecotypes.

Physical Attributes

If you look closely, you can distinguish between resident orcas and transient orcas by their physical characteristics alone. Resident orcas have rounded tips on their dorsal fins, whereas transient orcas have pointed dorsal fins. Also, resident orcas' fin tips are positioned further back than they are on transient orcas, and resident orcas often have some black areas on their dorsal saddles; transient orcas have uniformly gray dorsal saddles.

Geography

Transient orcas are found most often in the waters surrounding Alaska's Aleutian Islands and down to Southern California, and also along the coast of eastern Russia. "Their travel pattern tends to reflect a 'search mode,' often following the coastline, as they are stealthy hunters," Hoyt says.

The orca population in New Zealand is small with only about 200 whales. (Photo credit: Carrie Vonderhaar)Click to enlarge

Resident orcas are split into two communities, based on where they live. The northern resident community is typically found in the waters north of the top half of British Columbia's Vancouver Island, whereas the southern resident community is found in the waters south of the lower half of Vancouver Island. Resident orcas travel in a much more regular pattern than transient orcas. They usually stay within a 500-mile range of the coast.

Although transients and residents have different geographical ranges, Ken Balcomb, senior scientist at the Center for Whale Research, says the ranges overlap because transient orcas can be found worldwide.

Pod Size and Structure

Orcas, like other cetaceans, travel in pods. Orca pods are matrilineal family units, which means that they are headed by a female, usually the mother.

Resident pods are usually larger than transient pods. A resident pod can include from five to 50 orcas, whereas a transient pod usually consists of one to six orcas. The differences in pod sizes are probably due to differences in feeding habits (see "Feeding Habits" below).

Resident pods appear to be much more closely knit than transient pods. Hoyt has studied the resident orca populations of British Columbia, Alaska and Russia and says they are characterized by large extended families: "If the mother in a resident matrilineal group has a son, he will stay with her for his entire life, which is on average 29 years. If she has a female, then that female will move away to some extent when she has a calf."

This strong mother-offspring bond is not present in transient pods. "One, maybe two babies will stay with the mom, and the others will disperse," says Balcomb of transient orcas.

"The transient type pods are more fluid, breaking up and joining with other transients from time to time," Hoyt says. "It is completely different compared to the closeness we see day in and day out, year in and year out with the residents, who very much stay in their matrilineal groups with the mother as the focus."

Feeding Habits

Transient orcas prey mainly on marine mammals, such as sea lions, seals, porpoises, and even other dolphin species and gray whales. Hunting marine mammals, as transient orcas do, can be a demanding, time-consuming process, since they cooperate to sneak up on marine mammals. Pack-style hunting is easier in small numbers, which explains why transient pods are smaller than resident pods.

Resident orcas, on the other hand, feed mainly on fish, especially salmon, and have been seen swimming peacefully alongside dolphins and seals. Unlike their relatives who stalk marine mammals, resident orcas herd schools of fish to get their food; herding is most efficiently done with a large number of orcas, hence the comparatively larger size of resident orca pods.

Communication

Feeding habits also likely inform the differences between resident and transient orcas' communication strategies. Since transient orcas feed on marine mammals, which have acute underwater hearing, they have to approach very quietly in order to avoid being detected.

"Transients are generally more quiet," points out Balcomb. "They don't want to give themselves away to, say, a seal by echolocating." When transients use echolocation, they do so in single clicks.

On the other hand, salmon, which make up the majority of resident orcas' diets, have poor hearing, so resident orcas don't have to worry about being quiet and are more vocal, relying much more on echolocation to find their prey.

Among resident orcas of the Pacific Northwest, where Balcomb studies, each pod has its own set of complex call types, as opposed to transient pods in the same region, which communicate with simpler language and do not have different dialects.

Hoyt says that his research group, the Far East Russia Orca Project, is working hard to come to an understanding of how orcas build associations among themselves for mating and socializing versus for hunting.

"We're working hard on trying to understand the function of their calls and have discovered some new levels of relationships besides dialects that can show relatedness between communities within a clan. Understanding the true meaning and probable diverse function of the calls, which can vary completely from community to community around the world, would be an amazing achievement," he says.